Developed in concert with German firm Beck Automation, BMW's system is designed to use either new battery packs or "second-life" packs that have degraded too much for continued use in electric cars.

Battery packs that can no longer function in cars still have enough usable capacity for energy storage.

BMW has tested the concept over the past five years with various research projects, including a 2013 "micro-grid" project with the University of San Diego, and a 2014 collaboration in Germany with utility Vattenfall to use electric-car battery packs as buffer to help stabilize electricity grids.

General Motors, Nissan, and other makers have demonstrated "second-life" batteries in various similar projects, while Toyota provided used Camry Hybrid battery packs to store energy from solar panels at Yellowstone National Park.

BMW says it will offer the energy-storage batteries in 22-kilowatt-hour and 33-kWh sizes.

This corresponds to the battery-pack sizes of the current 2016 i3 and the larger-capacity version that will arrive for the 2017 model year.

Assuming daily energy usage of 15 to 30 kWh for a home—the U.S. national average is slightly over 30 kwh—BMW claims a pack can provide power for "a variety of appliances and entertainment devices" for up to 24 hours.

As with other energy-storage systems, however, the main benefit will likely be increased use of renewable energy.

Having a battery pack to store electricity from solar panels for later use allows homeowners to rely more on solar power, and buy less electricity from the grid.